This invention relates to the sensing of fluid parameters in wells, for example oil wells.
Oil wells usually have an outer casing down which is lowered a production string for the passage to the surface of the oil produced by the well. In certain wells it is necessary to have an artifical lift system to bring oil to the surface usually up the production tubing, either because of the lack of sufficient natural reservoir pressure to produce the well or to enhance production from a well that produces under natural pressure. A common system of artificial lift is to place an electrical submersible, or submergible, pump (ESP) down the well casing on the end of the production tubing to pump fluid from the casing up the production tubing. The installation of an ESP on the end of the production tubing eliminates the possibility of carrying out logging or other operations in the well below the production tubing with tools, unless a by-pass system is used to enable tools to be passed through the production tubing and into pump by-pass tubing by means of a Y-crossover sub, known as a Y-tool. Below the Y-tool there thus are two strings:
1) A pump string which produces the well, discharging fluid through the Y-tool into the production tubing to surface.
2) A by-pass tubing string which enables tools from the production tubing to go down past the pump and enter the well below the pump for logging or other operations.
When operating the well it is useful to know a parameter of fluid at the intake to the pump, pressure being the most common as it gives an idea of the fluid level in the well; and it is common practice to determine the value of the intake pressure by a sensor located at the bottom of the pump/motor assembly, power to the sensor and the signal from the sensor being carried by an electrical cable running to the surface, which may be the power cable. Other parameters such as temperature, flow or density may be similarly sensed.
It would also be useful to know a corresponding parameter on the discharge side of the pump, for example the pressure in order to give an indication of the efficiency of the pump.